Voltage regulators provide stable nearly constant (regulated) supply voltage to a load in an attempt to maintain the regulated supply voltage at the nearly constant value regardless of the current demands of the load. Voltage regulators are used in complex electronic systems to regulate supply voltages before being supplied to other circuit components. One of the many issues circuit designers must evaluate is how a circuit design behaves when power is first applied. Unexpected things can happen at startup. Capacitors must be charged, and all integrated circuits (ICs) change from an inactive state to an active state. Frequently, several voltage regulators provide power to the same circuit, and each regulator output must be sequenced at startup. Controlling the slew rate of a voltage regulator's output voltage at startup lowers the stress on circuit components and allows circuit designers to adjust the startup voltage rate to what is required by the circuit.
Complex electronic systems often require voltage regulators that provide soft start control of a voltage supply. Soft-start circuitry controls supply voltages at startup so that they rise at a controlled slew rate to an operating voltage. Soft-start circuitry can control inrush currents in capacitors, minimize load surges, and reduce the chances that the voltage supply will overshoot the operating voltage. Soft-start circuitry may take many forms. One particular implementation uses an error amplifier comprising an input pair of n-channel transistors. N-type transistors make it difficult to implement soft-start functionality because the input voltage to the n-channel transistors cannot swing too low. The input range of an n-type input amplifier cannot be close to its ground rail, and the feedback loop of the regulator using n-type transistors is only effective when the output voltage exceeds a certain threshold voltage.
A specific type of voltage regulator, called a “track regulator,” mirrors, or “tracks,” the output voltage of another voltage supply. In other words, the track regulator produces a secondary voltage supply that follows the voltage of a primary voltage regulator output. Track regulators are useful to create a second voltage supply with the same supply voltage of another regulator. Soft-start functionality is needed during the startup of a track regulator to limit inrush current and overshoot voltage. But it is difficult to implement the soft start of a track regulator when the error amplifier of a track regulator uses n-channel transistors. Again, the input to n-channel transistors cannot swing too low or be close to a ground rail in order for the track regulator to work properly, because the feedback loop of a track regulator with n-type transistors is only effective when the regulator output voltage exceeds the gate-to-source voltage of the n-channel transistors